It would be any key where every plaintext letter is shifted by a degree of half the alphabet, namely k = 'nnn...nnnn' where the period t = n. If you have a plaintext m ='agp', key k = 'nnn' and ciphertext c = 'ntc' then applying Enc_k(c) = 'agp' = c' = m because we shift all of the letters by the same degree forward to get the plaintext.